1. Field of the Invention
The present invention generally relates to a cell measurement apparatus and, more particularly, to a fuel cell measurement apparatus capable of measuring electric characteristics of a solid oxide fuel cell.
2. Description of the Prior Art
The solid oxide fuel cell (SOFC) is a clean energy conversion system, in which electrochemical reaction is used for power generation so as to convert chemical energy to electrical energy without combustion to prevent the environment from being polluted by carbon dioxide (CO2) and nitrogen oxide (NOx). Considering oil exhaustion and global warming, the solid oxide fuel cell has become a potential candidate as a clean alternative energy.
However, the solid oxide fuel cell has not been commercialized because the manufacturing cost is too high. Therefore, it is crucial to reduce its cost by material selection, cell design, or cell manufacturing.
Moreover, the measurement apparatus and technology for cell analysis, manufacturing quality control and commercialization evaluation are some important factors for developing the solid oxide fuel cell. Presently, there are two approaches for measuring electric characteristics and electrochemistry of a cell. One is by single cell measurement and the other is by multi-cell measurement. Considering multi-cell measurement, referring to S. C. Singhal, Solid State Ionics, 135 (2000) 305 and K. Ahmed, J. Gamman and K. Foger, Solid State Ionics, 152-153 (2002) 485, additional equipments are added to the system to integrate multiple cells as a cell stack and the system, which leads to very high cost.
The cell stack is assembled with time-consuming work, complicated elements, and high material cost. Moreover, the system is susceptible to the quality of these elements so that it is difficult to identify the manufacturing quality and explain the measured results to cause a mistake to mislead the research direction and hence extend the time to market. Therefore, it is advantageous to utilize single cell measurement to measure electric characteristics of the solid oxide fuel cell. Single cell measurement is a simple approach to the research of the solid oxide fuel cell. The leading institutes such as DOE, VTT, FZJ and ECN consider single cell measurement as an important step of cell quality control.
FIG. 1 is a cross-sectional view of a conventional fuel cell measurement apparatus (with reference to H. Peters and H. H. Mobius. Z. Physik. Chem., 209 (1958) 298.) Referring to FIG. 1, the conventional fuel cell measurement apparatus 100 is capable of measuring electric characteristics of the solid oxide fuel cell 50, which is a three-layer structure comprising an electrolyte layer 56 sandwiched between a porous anode layer 52 and a porous cathode layer 54.
More particularly, the solid oxide fuel cell 50 is attached onto the ceramic tube 110 using ceramic glue (not shown). The solid oxide fuel cell 50 is placed into the furnace 120 with the ceramic tube 110 to be heated up. The furnace 120 comprises two openings (not labeled) so that the ceramic tube 110 and the anode ceramic gas pipe 130 are inserted into the furnace 120 from the bottom and the top, respectively. The cathode ceramic gas pipe 140 is inserted into the ceramic tube 110.
When hydrogen H2 flows from the anode ceramic gas pipe 130 into the porous anode layer 52 and oxygen O2 flows from the cathode ceramic gas pipe 140 into the porous cathode layer 54, chemical reaction takes place in the solid oxide fuel cell 50 to generate electric energy. Moreover, in the prior art, a platinum conductive wire (not shown) is directly sintered on the porous anode layer 52 and the porous cathode layer 54. Electric characteristics of the solid oxide fuel cell 50 are measured using the platinum conductive wire (not shown).
However, since the solid oxide fuel cell 50 is required to be attached with a platinum conductive wire sintered thereon before electric characteristics are measured, related processing steps are time-consuming and material cost is increased. Moreover, even though ceramic glue can be used to combine the ceramic tube 110 and the solid oxide fuel cell 50 to achieve sealing. However, since the coefficient of thermal expansion of ceramic glue is not matched with that of the solid oxide fuel cell 50, cracks may appear in the solid oxide fuel cell 50. Moreover, in the prior art, after measurement, the ceramic glue has to be destructed before the solid oxide fuel cell 50 can be demounted. Therefore, the solid oxide fuel cell 50 cannot be re-attached onto the ceramic tube 110.
To overcome the foregoing problems, ProboStat (NorECs AS, Norway) discloses another fuel cell measurement apparatus as shown in FIG. 2A and FIG. 2B. For clarity, some elements are not shown in FIG. 2A. Instead, they are shown by dotted lines in FIG. 2B. Referring to FIG. 2A and FIG. 2B, the conventional fuel cell measurement apparatus 200 comprises a first current collecting unit 210, a second current collecting unit 220, a top holding set 230, a bottom holding set 240 and a spring 250. The top holding set 230 and the bottom holding set 240 are pulled by a spring 250 so that the solid oxide fuel cell 50 is clipped by the top current collecting unit 210 and the bottom current collecting unit 220. The top current collecting unit 210 and the bottom current collecting unit 220 prevent sintering and sealing on the solid oxide fuel cell 50 so that the solid oxide fuel cell 50 is re-usable.
More particularly, the first current collecting unit 210 comprises a first platinum conductive mesh 212 and a first platinum conductive wire 214. The first platinum conductive wire 214 is sintered on the first platinum conductive mesh 212. Similarly, the second current collecting unit 220 comprises a second platinum conductive mesh 222 and a second platinum conductive wire 224. The second platinum conductive wire 224 is sintered on the second platinum conductive mesh 222.
The first current collecting unit 210 and the second current collecting unit 220 are pressed against a porous anode layer (not labeled) and a porous cathode layer (not labeled) on the solid oxide fuel cell 50 for measuring electric characteristics of the solid oxide fuel cell 50. Moreover, the solid oxide fuel cell 50 is clipped by the first current collecting unit 210 and the second current collecting unit 220 using a top holding set 230, a bottom holding set 240 and a spring 250.
More particularly, the fuel cell measurement apparatus 200 further comprises an inner ceramic supporting tube 260 and an outer ceramic tube 270. The solid oxide fuel cell 50 is supported by the inner ceramic supporting tube 260 disposed inside the outer ceramic tube 270 to form two hermetically sealed rooms. To improve hermetic ness, in the prior art, a sealing washer 262 is disposed between the inner ceramic supporting tube 260 and the solid oxide fuel cell 50.
The bottom holding set 240 comprises a base 242 and a ceramic shaft 244. The inner ceramic supporting tube 260 is disposed on the base 242. The ceramic shaft 244 is disposed on the base 242 to support the second platinum conductive mesh 222 so that the second platinum conductive mesh 222 is tightly attached onto the solid oxide fuel cell 50. In the prior art, to keep the upward force in balance, a silicone tube 246 is further disposed between the base 242 and the ceramic shaft 244.
The top holding set 230 comprises a ceramic plate 232 and a ceramic shaft 234. The ceramic plate 232 presses the first platinum conductive mesh 212 downward so that the first platinum conductive mesh 212 is tightly attached onto the solid oxide fuel cell 50. The ceramic shaft 234 fixedly penetrates the ceramic plate 232 by use of a smaller ceramic shaft 236.
Moreover, the spring 250 is connected between the ceramic shaft 234 and the base 242 to pull the top holding set 230 and the bottom holding set 240. Accordingly, the top holding set 230 and the bottom holding set 240 tightly press the first platinum conductive mesh 212 and the second platinum conductive mesh 222 to clip the solid oxide fuel cell 50 to form two hermetically sealed rooms. In the solid oxide fuel cell 50, a current induced by chemical reaction can be introduced through the first platinum conductive mesh 212 and the second platinum conductive mesh 222.
Referring to FIG. 2A and FIG. 2B, the fuel cell measurement apparatus 200 further comprises a first gas pipe 280 and a second gas pipe 290. The first gas pipe 280 is connected to an opening (not labeled) in the ceramic plate 232 so that hydrogen H2 passes through the first platinum conductive mesh 212 to the solid oxide fuel cell 50, while the second gas pipe 290 penetrates the ceramic shaft 244 so that oxygen O2 passes through the first platinum conductive mesh 212 into the solid oxide fuel cell 50. Accordingly, the solid oxide fuel cell 50 is capable of converting chemical energy into electric energy that is introduced by the first platinum conductive wire 214 and the second platinum conductive wire 224.
However, the fuel cell measurement apparatus 200 has disadvantages such as:
1. The contacts between the solid oxide fuel cell 50 and the first current collecting unit 210 and the second current collecting unit 220 determine the contact resistances that strongly influence the output power of the solid oxide fuel cell 50. For the second current collecting unit 220, only the second platinum conductive mesh 222 is supported so that the contact area between the second current collecting unit 220 and the solid oxide fuel cell 50 is insufficient. Therefore, the output current is blocked and the cell performance is limited.
2. After hydrogen H2 and oxygen O2 flow from the first gas pipe 280 and the second gas pipe 290, the gas distribution is not uniform because there is no gas path. As a result, the energy generated by the solid oxide fuel cell 50 is not uniform and thermal stress occurs due to non-uniform temperature distribution, which leads to cracks in the solid oxide fuel cell 50.
3. The second platinum conductive mesh 222 is often out of shape and destructed because the ceramic shaft 244 supports the second platinum conductive mesh 222. Therefore, the second platinum conductive mesh 222 cannot be re-used. Moreover, if ceramic shaft 244 is not disposed properly or obliquely supports the second platinum conductive mesh 222, the solid oxide fuel cell 50 may be damaged.
4. In the prior art, the pulling force of the spring 250 is a constant that cannot be adjusted. Therefore, when the thickness of the solid oxide fuel cell 50 varies, the applied pulling force from the spring 250 will be different, which leads to poor condition consistency of cell measurement. Moreover, in the prior art, the pulling force of the spring 250 is a constant. If the mechanical strength of the tested solid oxide fuel cell 50 varies, it may cause damage to the solid oxide fuel cell 50 since the pulling force of the spring 50 is too large when solid oxide fuel cell 50 is installed or measured. On the contrary, if the pulling force of the spring 250 is too small, the first current collecting unit 210 and the second current collecting unit 220 cannot tightly contact the solid oxide fuel cell 50 to result in larger resistance. Moreover, since the base 242 is hooked by the spring 250, there is risk that the hook may be loosen so that the ceramic shaft 234 or even the solid oxide fuel cell 50 cracks due to collision. In this case, the solid oxide fuel cell 50 cannot be demounted and re-used.
5. The first gas pipe 280 and the second gas pipe 290 are not equipped with a pressure meter so that the anode pressure and the cathode pressure cannot be monitored. Therefore, gas leakage occurs due to unbalanced pressure and error in measurement results from non-uniform reactive gas concentrations.